The present invention relates to a transmitter/receiver having a superheterodyne receiver and, more particularly, to a transmitter/receiver which prevents the modulation accuracy of a signal to be transmitted and reception sensitivity from being degraded even when a reference signal fed from the outside involves noise.
There has been known a transmitter/receiver made up of a superheterodyne receiver, a transmitter, a local oscillation signal generating section for transmission, and a first and a second local oscillation signal generating section for reception. The local oscillation signal generating sections are each implemented as a phase locked loop (PLL) frequency synthesizer. The PLL synthesizer has a voltage controlled oscillator (VCO), a variable frequency divider for sequentially dividing the output frequencies of the VCO, a phase comparator for comparing the phase of the output of the frequency divider with that of a reference signal, and a low pass filter for smoothing the output of the phase comparator to thereby produce a control signal. Controlled by this control signal, the VCO outputs a signal synchronous to the reference signal. The transmission and first reception local oscillation signal generating sections are each implemented by a PLL capable of switching the frequency rapidly, i.e., having a high loop gain since it has to change the frequency in association with a transmission frequency or a reception frequency. Conversely, the second reception local oscillation signal generating section is implemented by a PLL having a low loop gain since the frequency thereof needs only to be constant.
The conventional transmitter/receiver has a problem left unsolved, as follows. When the reference signal from the outside involves noise, the transmission and first reception local oscillation signal generating sections cannot remove the noise due to their high loop gains. As a result, the transmission and first reception local oscillation signal s from such sections are unstable and low in carrier-to-noise (C/N) characteristic, degrading the modulation accuracy of a signal to be transmitted as well as a reception characteristic.